(1) Field of the Invention
The invention concerns an aluminum alloy for diecasting of components with high elongation in the cast state.
(2) Prior Art
Diecasting technology has today developed so far that it is possible to produce components with high quality standards. The quality of a diecasting however depends not only on the machine setting and the process selected but to a great extent also on the chemical composition and the structure of the aluminum alloy used. The latter two parameters are known to influence the castability, the feed behavior (G. Schindelbauer, J. Czikel “Mould filling capacity and volume deficit of conventional aluminum diecasting alloys”, Giessereiforschung 42, 1990, p. 88/89), the mechanical properties and—particularly important in diecasting—the life of the casting tools (L. A. Norström, B. Klarenfjord, M. Svenson “General Aspects on Wash-out Mechanism in Aluminium Diecasting Dies” 17th International NADCA Diecasting Congress 1993, Cleveland, Ohio).
In the past little attention has been paid to the development of aluminum alloys which are particularly suited for diecasting of high quality components. Manufacturers in the car industry are now increasingly required to produce e.g. weldable components with high ductility in the diecasting process, since diecasting is the most economic production method for high quantities.
The refinement of the diecasting technology now allows the production of weldable components of high quality. This has expanded the area of application for diecastings to include chassis components.
Ductility is increasingly important, in particular in components of complex design.
In order to achieve the required mechanical properties, in particular a high elongation to fracture, the diecastings must usually be subjected to heat treatment. This heat treatment is necessary for forming the casting phase and hence achieving ductile fracture behavior. Heat treatment usually means solution annealing at temperatures just below the solidus temperature with subsequent quenching in water or another medium to temperatures <100° C. The material treated in this way now has a low elongation limit and tensile strength. In order to raise these properties to the required value, artificial ageing is then performed. This can also be process-induced e.g. by thermal shock on painting or stress-relief annealing of a complete assembly.
As diecastings are cast close to the final dimensions, they usually have a complex geometry with thin walls. During the solution annealing, and in particular the quenching process, distortion must be expected which can require retouching e.g. by straightening the casting or, in the worst case, rejection. Solution annealing also entails additional costs, and the efficiency of this production method could be substantially increased if alloys were available which fulfilled the required properties without heat treatment.
An AlSi alloy with good mechanical values in the casting state is known from EP-A-0 687 742. Also for example EP-A-0 911 420 discloses alloys of type AlMg which in the casting state have a very high ductility, but with complex form design however tend to hot or cold cracking and are therefore unsuitable. A further disadvantage of ductile diecastings is their slow ageing in the cast state which can lead to a temporary change in mechanical properties—including a loss of elongation. This behavior is tolerated in many applications as the property limits are not exceeded, but cannot be tolerated in some applications and can only be excluded by targeted heat treatment.